The present invention relates to a method for reducing gas pressure in an electrochemical cell and, more particularly, to a method for treating carbon cathodes of a primary electrochemical cell in order to reduce the gas pressure in the cell during open circuit and during discharge of the cell.
Primary electrochemical cells are generally well known and understood by those skilled in the art. One particularly useful primary electrochemical cell, especially for high current drain applications, is a so-called prismatic primary electrochemical cell. Such cell is generally described in U.S. Pat. No. 4,086,397, in the names of Franz Goebel and Nikola Marincic, and includes a physically large battery stack enclosed together with an electrolytic solution within a large, generally rectangular, metal (e.g., stainless steel) housing. The battery stack as used within the cell comprises a large number of generally rectangular cell components including a plurality of anodes, carbon cathodes, and insulative separators (e.g., of glass fiber) between the anodes and the carbon cathodes. Each anode generally comprises a large rectangular sheet of an oxidizable alkali metal, such a lithium, physically impressed into a supporting metal (e.g., nickel) grid, and each carbon cathode comprises a metal (e.g., nickel) current collector grid and an aggregation of porous, semi-rigid carbon globules or conglomerates physically impressed into the grid. A common and preferred electrolytic solution employed in the cell as described above is an electrolyte solution comprising a reducible soluble cathode material such as thionyl chloride and an electrolyte solute such as lithium tetrachloroaluminate dissolved in the thionyl chloride.
By the appropriate selection of the battery cell components and materials, a cell as described above can be constructed to have any one of several possible sizes and characteristics. A typical cell, for example, has exterior dimensions of approximately 18 inches.times.13 inches.times.10 inches, a weight of 156 pounds, a capacity rating of 10,000 ampere-hours, and a nominal discharge current of 40 amperes.
During open circuit or during discharge of a primary electrochemical cell having the above-described characteristics, various gases are present in the cell, either initially or generated subsequent to closing (i.e., sealing) of the cell. The quantity of these gases generally increases with time, leading to an increase in gas pressure within the cell. For reasons of structural integrity of the cell and minimizing damage thereto, it is ordinarily desirable to minimize the gas pressure within the cell. The gases present within the cell during the cell lifetime (open circuit and discharge period) include thionyl chloride vapors and small amounts of insoluble gases such as oxygen, nitrogen, argon or helium present in the cell from the start, and gases generated during the cell lifetime, such as hydrogen and sulfur dioxide. The oxygen and nitrogen gases are present in the cell throughout the manufacture of the cell. The inert argon or helium gas is employed to displace air in the cell before it is activated by the addition of the electrolytic solution, at which time the cell is evacuated to remove as much of the gases as possible, leaving only small amounts of the gases in the cell. The hydrogen generated in the cell during discharge is due to the presence of trace amounts of water in the cell which react with the electrolytic solution to produce a variety of protic (proton-containing) hydrolysis products which, in turn, react with lithium metal to produce hydrogen gas.
The water in the cell which leads to the generation of hydrogen gas as discussed above is normally present within the carbon cathodes as a result of an affinity of the water for, and absorption by, these electrodes. Obvious solutions for reducing or minimizing the amount of water absorbed by the carbon cathodes, such as heat and vacuum treatments of the carbon cathodes, are of limited value because of the extremely low vapor pressure of absorbed water. The preparation of extremely dry carbon cathodes is therefore very difficult.